Nanocrystalline cellulose is produced by the controlled acid hydrolysis of cellulose sources such as bleached wood pulp [1-4]. The use of sulfuric acid imparts negatively charged, acidic sulfate ester groups at the NCC surface, resulting in stable aqueous suspensions due to electrostatic repulsion between the colloidal NCC particles [3, 5-10].
NCC is a renewable, recyclable, carbon neutral material. These factors and potentially unique mechanical and optical properties of NCC have generated great interest in manufacturing NCC-based products at an industrial scale. However, because NCC is initially produced as an aqueous suspension with only a few weight percent solids content, any high-volume application will require NCC to be delivered in dried form and resuspended at the site of use in order to minimise both cost and shipment size and weight. Drying NCC also provides another benefit by preventing bacterial and fungal growth, to which aqueous NCC suspensions are susceptible when stored for long periods, even at 4° C.
Drying is also a necessary step in the removal of water from NCC suspensions for solvent exchange prior to redispersing NCC in organic solvents [12-13] for chemical modification, and in polymers for nanocomposites manufacture [1]. Freeze-drying is generally used to accomplish this. Often, additives and chemical surface modification have been used to aid in the redispersion of NCC particles in organic solvents. Microfibrillated cellulose produced without chemical modification has been dispersed in polar solvents such as glycerine, poly(ethylene glycol) and DMSO [14]. Stable suspensions of cellulose whiskers (crystalline cellulose similar to NCC but microns in length) prepared from marine invertebrates have been obtained in toluene and cyclohexane using a phosphoric ester surfactant [13]. Partial surface silylation has also been used to disperse NCC in nonpolar organic solvents [15] and acetone [16]. Finally, grafting low molecular weight poly(ethylene glycol) onto the surface of cellulose nanocrystals has been found to yield stable suspensions in chloroform [17].
Several attempts to redisperse freeze-dried cellulose whiskers and NCC in polar organic solvents such as DMF and DMSO without surfactants or chemical modification have been successful [12,18]. Dilute suspensions were prepared by vigorous mixing and intensive ultrasonication of the dried cellulose nanocrystals in the organic solvents.
Nanocrystalline cellulose suspensions produced by sulfuric acid hydrolysis are not dispersible in water once they have been fully dried to solid films, even under fairly gentle drying conditions, for example in a vacuum oven at 35° C. for 24 hours [11]. It is thought that the proton counterions contributed by the acid and associated with the sulfate groups imparted to the NCC during hydrolysis are responsible for strengthening the intermolecular hydrogen bonding between the cellulose crystallites and causing the NCC film's non-redispersibility [11]. The proton counterions can be exchanged for other monovalent counterions; dried NCC with, e.g., sodium counterions was found to be completely redispersible in water [11]. Based on FT-IR spectra of acid-form (H-NCC) and sodium-form (Na-NCC) NCC films, it has been suggested that extra intermolecular hydrogen bonding between cellulose nanocrystals in the H-NCC film may prevent its redispersion in water [11].
It is, however, known that freshly cast free-standing H-NCC films or thin H-NCC films spun onto solid substrates will swell and disperse in water with slight agitation [8,19,20]. The moisture content of these films was not determined. There is no prior art (journal articles, patents, etc.) regarding the effect of moisture content or humidity on the dispersibility of dried NCC suspensions.
Cellulose is a hygroscopic material and will absorb moisture from the surrounding air; it has been found from moisture sorption isotherms that cellulose samples of differing crystallinity will absorb different quantities of moisture, higher crystallinity resulting in lower final moisture contents [21]. Sorption calorimetry studies on microcrystalline cellulose (MCC), ball-milled cellulose (of lower crystallinity) and cellulose recrystallized after ball-milling have also found that the most crystalline sample (MCC) showed the lowest water uptake [22]. The authors suggested that in addition to adsorbing at the cellulose-air interfaces, near-monolayers of water molecules adsorbed between the solid interfaces of cellulose microfibrils in the MCC powder, followed by additional layers. As NCC films can be described as having an “open structure” not dissimilar to that of MCC powder, containing ordered crystalline elements with spaces between them, adsorption of water molecules between the nanocrystal surfaces may partially explain the mechanism of the effect of moisture content on their dispersibility (see FIG. 1). FIG. 1 (based on Figures in [21] and [22]) shows a schematic diagram of water molecules B adsorbing on cellulose surfaces; the rectangular rods A can represent microfibrils in the case of MCC, or individual cellulose nanocrystals in the case of solid NCC films. Highly crystalline but non-porous algal cellulose extracted by HCl hydrolysis of green algae has been found to exhibit greater N2 adsorption than H2O adsorption, in contrast to porous cellulose powders with crystalline elements, which displayed much greater H2O than N2 adsorption, suggesting that water adsorbs between the solid surfaces (e.g., microfibrils) of the porous cellulose [21]. It has been found previously that water adsorbs onto the crystalline surfaces of cellulose [23].
Previous studies of the dispersibility of dried H-NCC have been confined to its dispersibility in organic solvents and polymers for chemical surface modification and nanocomposites [1,12]. H-NCC can also be easily converted to a more “permanent” non-dispersible form if so desired, using a simple drying step. In addition, lower chemical costs and minimal manipulation required make dispersible dried H-NCC an attractive option.
When the proton counterion is exchanged for a variety of monovalent cationic counterions, including Na+, K+, Li+, NH4+ and tetraalkylammonium (R4N+), protonated trialkylammonium (HR3N+), protonated dialkylammonium (H2R2N+), and protonated monoalkylammonium (H3RN+) ions, the air-dried solid NCC films produced from these suspensions are completely redispersible in water [11]. After brief sonication, the resulting colloidal NCC suspensions were found to have properties similar to those of the native suspensions [11]. These suspensions underwent phase separation to give two phases within several hours of standing, which is an indication of a well-dispersed suspension. The neutral forms of NCC such as Na-NCC possess an advantage over the acidic H-NCC: The freeze-drying process causes almost immediate partial desulfation of H-NCC (removal of the anionic sulfate ester groups which contribute to NCC suspension stability); this process continues during storage of the freeze-dried H-NCC, accompanied by degradation of the is cellulose chains. Neutral Na-NCC does not undergo this degradation when freeze-dried.